The present invention relates to a near-infrared communication apparatus for transmitting an optical signal through space.
As is well-known, a near-infrared communication apparatus for transmitting an optical signal through space does not require a provision of an optical fiber and its installation is easy. Because of this advantage, the near-infrared communication apparatus is being widely used for links for connecting two points and for LANs (local area networks).
According to the conventional near-infrared communication apparatus, optical transceivers 11 and 12 are installed on the ceiling and on the table respectively as shown in FIG. 6. The optical transceiver 11 installed on the ceiling has a plurality of LEDs (light emitting diodes) 111 for carrying out transmission and a PD (photodiode) 112, and they are nondirectional as a whole. In the optical transceiver 12 installed on the desk, a transmission light emitted from an LED 121 is converted into substantially collimated lights by a concave mirror 122 and is outputted to the optical transceiver 11 on the ceiling. The light transmitted from the transceiver 11 to the ceiling is collected to a photodiode 123 by a concave mirror 124. With the above-described optical system, it is possible to carry out communications between the transceivers 11 and 12 to about 10 Mbps.
There is also a near-infrared communication apparatus for emitting a transmission light to the ceiling and for receiving the light diffused on the ceiling, without the provision of a transceiver on the ceiling. According to this system, however, the power of the received light becomes weak so that an optical signal can be transmitted at only a low speed of about 1 Mbps.
When the case of increasing the transmission speed to about 100 Mbps is considered, this realization is not easy because it is difficult to achieve an LED of both high speed and high output. Thus, the use of a high-output LD (laser diode) in stead of an LED is considered. However, the LD has a high coherency and therefore it is necessary to take into consideration bad influence that can be observed by human eyes.
In order to avoid the above problem, a method for reducing spatial coherency with a diffusing plate using a holography has been proposed. However, even if the holography is used, the light power intensity must be kept less than 100 mW/cm2. Further, when a high-output laser is used, a beam of a large diameter must be formed, with the result that the apparatus becomes bulky. Moreover, the holography of a large diameter has a problem of mass production and thus this is expensive.
Further, according to the above-described prior-art example of installing the optical transceiver on the ceiling, it is necessary to make arrangement for preventing signal lines and current lines from falling from the ceiling, which requires a troublesome installation work.
Further, under the trend of increasing need for a higher speed communication system, high attention has been paid to IrDA (Infrared Data Association) devices and diffusion type LANs for the purpose of increasing the data transmission speed.
In order to achieve high-speed data transmission of above 50 Mbps or 100 Mbps and to achieve compact transceivers, a laser diode is effective as a photoemissive device in the light of the fact that the laser diode can perform high-speed modulation and is highly efficient. However, the laser diode has a problem of high spatial coherency of an emitted beam while having the above-described advantages. Accordingly, from the viewpoint of the protection of the human body, a method of once diffusing an output light from the laser diode, or making the laser beam incoherent, and then radiating the diffused light to the space is known as described, for example, in the IEEE Photonics Technology Letter, VOL. 6, No. 10, pp 1268-(1994), "50-Mb/s Diffuse Infrared Free-space Link Using On-off Keying With Decision-Feedback Equalization", (Gene W. March et al.).
A conventional optical spatial transmission apparatus is structured by an optical diffusing plate 171, a laser 172, a photoreceptor 173, an optical system 174 made of an optical filter and a lens, a receiving circuit 175 and a driving circuit 176, as shown in FIG. 12.
Data transmitted from the driving circuit 176 is inputted to the laser as a modulation signal. An optical signal outputted from the laser 172 is diffused by the optical diffusing plate 171 and is then radiated to the space. The optical signal radiated to the space has its disturbance light removed by the optical system 174 made of the filter and the lens. The optical signal is then collected by the photoreceptor 173 and is then photoelectrically converted. The photoelectric signal is guided to the receiving circuit 175. The light of the laser is made incoherent by using the optical diffusing plate 171 as described above, to thereby secure the safety of the human body.
The optical diffusing plate to be used by the above-described conventional optical spatial transmission apparatus is structured by a glass plate 181 and an optical diffusion area 184, as shown in FIG. 13. The optical diffusing plate 184 includes a frost-type glass of which surface is frosted by using an abrasive and an opal-type glass on the surface of which a fine-particle optical diffusing substance is dispersed.
The magnitude of the dispersion of these optical diffusing plates and optical loss have mutual opposite effects. In other words, when the diffusion effect of the optical diffusing plate becomes large, a laser beam can be made incoherent sufficiently to ensure safety of the human body, but this causes a larger optical loss. Therefore, when the priority is placed on the safety of the human body, the optical loss becomes larger so that it is not possible to achieve an increase in the data transmission speed or it is not possible to expand the cover area.
Further, since the beam pattern of the laser beam is oval, an image formed by the lens on the light receiving side also becomes oval so that the photoreceptor and the light receiving surface do not match sufficiently. In other words, the area of the light receiving surface onto which an optical signal is effectively emitted to the photoreceptor becomes a part of the light receiving surface, and thus the stray capacity of the photoreceptor becomes larger than is necessary. Accordingly, the data transmission speed of the data that can be received becomes low. Moreover, when the light of a part of the oval is received on the light receiving surface, the light receiving power becomes smaller. Therefore, when the received light and the light receiving surface do not match satisfactorily, the increasing of the data transmission speed is interfered.